US senators have lambasted homeland security chief Michael Chertoff for his department's response to Hurricane Katrina last August.
Chertoff said he would have done some things differently
A Senate committee, with members from both parties, said his agency's response "must be judged a failure".
For his part Mr Chertoff admitted "many lapses" in the action his agency took.
The hearing comes days after the leaking of a Congressional report which described the government response as marked by "fecklessness and flailing".
More than 1,300 people across five states were killed by Hurricane Katrina. Hundreds of thousands of people were displaced in its aftermath.
The head of the Senate Committee on Homeland Security and Governmental Affairs, Republican Senator Susan Collins, called the homeland security department's performance "late, uncertain and ineffective".
The top Democrat on the committee, Senator Joseph Lieberman, castigated Mr Chertoff for his decision to attend a bird flu conference in Atlanta the day after the hurricane hit - instead of heading straight to New Orleans.
"How could you go to bed that night [29 August] not knowing what was going on in New Orleans?" Mr Lieberman asked.
"[I]t was my belief... the storm had not done the worst that could be imagined," Mr Chertoff responded.
Hurricane Katrina breached the levees protecting New Orleans
However, Mr Lieberman said that under Mr Chertoff's supervision, emergency workers "ran around like Keystone Kops, uncertain about what they were supposed to do or uncertain how to do it".
The Federal Emergency Management Agency (Fema), which is run by the Department of Homeland Security (DHS), has been severely criticised for a slow and chaotic response in the days after the disaster.
Much of the vilification has been reserved for the then Fema head Michael Brown.
His suitability as Fema chief has been questioned, with critics pointing to his previous career as a lawyer and commissioner for the International Arabian Horse Association, a breeders and horse show organisation.
At the hearing Ms Collins said she remained "perplexed" about Mr Chertoff's decision to choose Mr Brown to lead the Katrina response.
Mr Chertoff reacted that he had had "no reason to doubt his [Mr Brown's] commitment" at the time.
However, he added: "if I knew then what I know now about Mr Brown's agenda, I would have done something different".
"There are many lapses that occurred, and I've certainly spent a lot of time... thinking about things that might have been done differently," Mr Chertoff said.
Last week Mr Brown, who was forced to quit Fema over his handling of the disaster, also testified before the Senate committee.
He admitted officials did not act swiftly enough, but he said the DHS had focused unduly on the threat of terrorism - at the cost of preparation for a natural disaster.
The Senate committee hearing came on the day a leaked Congressional report which singles out Mr Chertoff for criticism was released.
"At every level - individual, corporate, philanthropic and governmental - we failed to meet the challenge," the summary of the 600-page report said.